1. Field of the Invention
This invention relates to a method of cementing a conductor pipe in offshore operations and particularly to methods for use in wells in water over 1000 feet deep where the surface mud is not well consolidated.
2. Description of the Prior Art
Well completions in water over 1000 feet deep often require special techniques to install conductor casing. Well completions at depths in excess of 2000 feet of water are often referred to as "deepwater" operations. In deepwater operations, the formations where conductor pipe is cemented that is less than 2000 feet below mud line (BML) are generally young geologically and are not well consolidated. The formations generally are the product of erosion from the continental shelf. This can cause either of two problems in cementing. The formation may be so weak that it may fracture during cementing and cause the loss of cement into the formation. Alternatively, the formation may experience high saltwater or other fluid flow through the formation resulting in fluid influx.
In a typical conductor pipe installation, a 24-30 OD inch surface pipe is driven at least 200 feet BML. A large diameter (20 in. OD when 30 in. OD used) conductor pipe then is cemented by the conventional innerstring method through the drill pipe, with cement returns back to the ocean floor. Since no riser is used, the annular returns must be taken at the sea floor. Cool temperatures caused by the seawater typically slow the cement hydration process and extend the transition time of the cement slurry which allows fluid influx to begin. The term "transition time" refers to the period of time between the onset of hydration of the cement and the development of compressive strength wherein the gel strength increases to a level of about 500 lbf/100 ft.sup.2 whereby fluid migration is substantially prevented. During the transition time, a fluid such as oil, gas or water can migrate through the setting cement slurry forming channels that effect the integrity of the cement sheath. The fluid migration is possible during transition because the cement column in the well bore begins to support itself and stops exerting hydrostatic pressure on the fluid surrounding the well bore. When the exerted hydrostatic pressure falls below the formation fluid pressure, migration can occur and will continue until the cement develops sufficient compressive strength to prevent further migration.
In some instances, the formation sands may be over-pressured by water so that water or other formation fluids flows into the setting cement sheath during the transition time. Prevention of such flow is critical to a successful cementing job and to avoid expensive remedial squeeze cementing treatments. Containment of the over-pressured formation fluid often is complicated by weak zones in the formation that can fracture due to the fluid pressure of the cement slurry. If a fracture is formed, the cement slurry can flow into the fracture and be lost from the well bore.
One method that has been utilized in the past has involved lightening the cement slurry by the addition of mix water. Such slurries have little useful strength at slurry densities below 11 lb/gal and have long transition times because of the cool formation temperatures. Water-extending a cement slurry as accomplished by the addition of water and an extending material such as sodium silicate or bentonire to the normal cement slurry. The amount of water-extending material and water added to the normal weight cement slurry depends on the final desired cement slurry density and the requirement of little or no free water in the cement slurry.
The use of water-extended cement slurries in deepwater completions has resulted in numerous well problems due to the cemented annulus being highly contaminated by formation fluids whereby the cement sheath can not adequately support the conductor string. This can result in casing buckling and loss of the well.
It would be desirable to provide a method by which a cementing operation can be performed on a conductor casing in over-pressured, poorly consolidated formations.
It would be desirable to shorten the transition time of the cement while making the drilling mud and cement slurry weight compatible with the formation fracture gradients to avoid cement losses to the formation.